A hybrid vehicle may include an engine and an electric machine to provide torque to propel the vehicle. The electric machine may operate more smoothly at low speeds as compared to the engine. Therefore, it may be desirable to operate the hybrid vehicle in an electric only mode where the electric machine is the sole torque source when the vehicle is operated at low speeds. One operating mode where vehicle speed is low is creep mode. Creep mode may be a mode where driver demand torque is zero or a small torque less than a threshold torque, vehicle speed is less than a threshold speed, and vehicle brakes are not applied. During creep mode, a torque source (e.g., an engine and/or an electric machine) may supply a small amount of torque to allow the vehicle to creep at a slow speed (e.g., less than 8 KPH) or to hold the vehicle stationary on a small positive incline. It may be desirable to transition the torque source between speed control mode and torque control mode while the vehicle is in creep mode based on vehicle operating conditions. However, if torque from the torque source is not supplied smoothly in the transition from speed control mode to torque control mode, a driver may experience undesirable vehicle motion while the vehicle is creeping. Consequently, the driver may not experience smooth vehicle motion during creep.
The inventors herein have recognized the above-mentioned disadvantages and have developed a driveline method, comprising: adjusting torque of a torque source in response to a virtual torque converter impeller speed when a torque converter clutch is locked.
By adjusting torque output of a torque source in response to a virtual torque converter impeller speed, it may be possible to provide the technical result of improving transitions of the torque source from speed control mode to torque control mode during a vehicle creep mode. For example, an electric machine may be a torque source for a hybrid vehicle. The electric machine may be operated in a speed control mode while a vehicle is in a creep mode and while a torque converter clutch of a torque converter receiving torque from the electric machine is in an open state. The electric machine may be transitioned from the speed control mode to a torque control mode when the torque converter clutch is locked during the vehicle creep mode to improve driveline efficiency. Electric machine torque may be adjusted during the transition based on a virtual torque converter impeller speed. The virtual torque converter impeller speed is an input into a torque converter model that outputs torque converter impeller torque. The electric machine torque is adjusted to maintain the torque converter impeller torque so that driveline torque disturbances may be reduced.
The present description may provide several advantages. Specifically, the approach may improve driveline speed control mode to torque control mode transitions. Further, the approach may provide for reduced driveline torque disturbances. Further still, the approach may reduce degradation of driveline components.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.